Method of treating alloy steel



Sept. 30, 1969 Filed July 5. 1966 CHARPY IMPACT VALUE AT-50 "C,kg-m/;mCHARPY IMPACT VALUE AT-f':v0C,kgm/cm KAZUHISA SUZUKI METHOD OF TREATINGALLOY STEEL 4 Sheets-Sheet 1 o l x l I 1 0 200 400 600 800 I000 I200I400 TEMPERATURE,C

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METHOD OF TREATING ALLOY STEEL Sept. 30, 1969 Filed July 5, 1966 4Sheets-$heet 2 I I l COOLING TIME, SECONDS TEMPERATURE, C

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a. a: I U o l l l A 1 1 0 2O [4O 60 GO I00 I20 TIME, SECONDS UnitedStates Patent US. Cl. 148127 Claims ABSTRACT OF THE DISCLOSURE The steelis heated to a temperature above the A transformation point and is thencooled to approximately the temperature at which martensitictransformation of the steel begins, along a cooling curve approximatingthe cooling curve resulting from the welding thermal cycle, after whichthe steel is cooled more slowly.

This invention relates to a method of treating alloy steel.

The conventional manufacture of alloy steels of high strength includes aheat treatment comprising quenching 'ice alloy steel to the temperatureto which the steel is subsequently heated, for example, by a weldingoperation;

FIG. 2 is a plot relating the notch toughness of the heat affected zoneof the same alloy steel as of FIG. 1 to the time interval in which thesteel is cooled from a temperature of 800 C. to 500 0.;

FIG. 3 is a plot relating the cooling of another alloy steel to thephases thereof;

FIG. 4 is a plot indicating various cooling rates of the steel of FIG. 3after it has been subjected to submerged arc welding;

FIG. 5 is a plot superimposing upon the plot of FIG. 3 a portion of theband included by the extreme members of the set of curves of FIG. 4;

FIG. 6 is a plot comparing the notch toughness of conventional alloysteel to another alloy steel treated according to the present invention,at various temperatures;

FIG. 7 is a plot superimposing upon the FIG. 1 plot a like plot for thealloy steel of FIG. 6, and

FIG. 8 is a plot superimposing upon the plot of FIG. 2 a like plot forthe alloy steel of FIG. 6.

Examples of two conventional high strength alloy steels are set forth inTable 1.

TABLE 1 Type Heat treating C Si Mn P S Ni Or A 870 0. WQ, 050 C. no-.-"16 .38 .98 .01 .016 .95 .47 B 870 C. WQ, 650 0. AG... .15 .32 1.05 .007.018 .93 .56

Charpy im- Yield Tensile Elon- Reduction pact value point strengthgation of area, 50 C. Type Mo B (kg/mm?) (kg/mm?) (percent) (percent)(kgmJcmfi) the hot rolled ingot. The purpose of the heat treatment isUnder the heat treating column is indicated a conto increase the tensilestrength and toughness of the alloy ventional heat treatment. When theseconventionally steel. The alloy steels are steels containing one or morealloying elements. Typical alloying elements are silicon, manganese,nickel, chromium, molybdenum, boron and the like. When the alloyingelements constitute about 2% or more by Weight of the steel the steelwhen conventionally heat treated has a tensile strength on the order of70 to 100 kg./mm. and is known as a high strength alloy steel. However,in spite of their great initial toughness it is found that when thesehigh strength alloy steels are subjected to welding, particularly highheat input welding such as automatic welding, the heat of the weldingseverely decreases the notch toughness of the steel.

It is an object of the present invention to provide a method of treatingalloy steel, especially, but not only, high strength alloy steel, forthe purpose of substantially mitigating or essentially eliminating adecrease therein of toughness upon subjection to a welding operation.

Briefly, the method of the present invention involves a heating of thesteel followed by a cooling thereof at a rate approximating the rate atwhich the steel will be cooled after the welding operation.

Other objects and features of the present invention will become apparentfrom the following detailed description and accompanying drawings, inwhich drawings:

FIG. 1 is a plot relating the notch toughness of the heat affected zoneof a conventionally treated high strength treated alloy steels arewelded it is found that there is a subsantial decrease in their notchtoughness in areas thereof which have been heated by the weldingoperation, especially areas which have been heated to temperatures atwhich substantial austenite formation takes place. Thus, for example,when the type B steel of Table 1 is heated to temperatures up to 1350C., as would occur in the case of welding, and during subsequent coolingis allowed to cool from 800 C. to 500 C. during a period of about 25 to28 seconds a substantial decrease in the notch toughness thereof, asindicated by the Charpy impact value, occurs (FIG. 1). Thus, the Charpyimpact value of the area of the type B steel which has been heated to atemperature of 1350 C. has been lowered to 1 kg. m./cm. from an initialvalue for the type B steel of 7.6 kg. m./ cm.

Furthermore, the amount of decrease in the Charpy impact value increasesup to a certain point as the time interval for cooling from 800 C.(i.e., about the A transformation temperature) to 500 C. (i.e., about atemperature at which transition from austenite to martensite begins) isincreased. Thus, for example, in the case of the type B steel the Charpyimpact value rapidly decreases as the time interval for cooling from 800C. to 500 C. is increased up to about 40 seconds (FIG. 2). Such timeintervals for cooling from 800 C. to 500 C. are not uncommon afterwelding.

In the prior art it was attempted to prevent or mitigate this decreasein notch toughness by subjecting the steel to a preliminary heattreatment as indicated in Table 1 or to a post-welding heat treatment.As indicated above, a severe decrease in the notch toughness of thesteel upon welding cannot be eliminated by the preliminary heattreatments of the prior art. Post-welding heat treatments also have notbeen particularly successful. According to the present invention, it hasbeen found that by subjecting the steel to a preliminary cooling cycleapproximating the cooling cycle which the steel will be subjected towhen it is welded and subsequently cooled essentially eliminates anysignificant decrease in the notch toughness of the steel upon weldingand cooling. Furthermore, it has been .found that the alloy steel mayhave a relatively low alloy content and, nevertheless, be susceptible tothe treatment of the invention.

An example of a steel having a tensile strength of 70 kg./mm. but,nevertheless, having a relatively low alloy element content is onecontaining by weight 0.14% C, 0.25% Si, 1.27% Mn, 0.53% Ni, 0.24% Cr,and 0.19% Mo. In the phase diagram of this steel (FIG. 3), I indicatesthe austenite zone, II indicates the ferrite zone, III indicates thepearlite zone, IV indicates the transitional zone between pearlite andmartensite, and V indicates the martensite zone. The steel has an Atransformation point of approximately 710 C. and an A transformationpoint of approximately 810 C. According to the invention, the steel isheated to a temperature immediately above the A transformation point andstarting from a time designated zero is cooled as indicated by a coolingcurve I. The steel is rather rapidly cooled to the initial point ofmartensite transformation M (about 400 C.) and, thereafter, iscontinuously cooled down through the termination point of martensitetransformation M (about 220 C.) and down to room temperature. A rate ofcooling between the A transformation point and the starting point ofmartensite transformation M is provided which will approximate thepost-welding cooling rate between those temperatures.

Four samples of the steel which have been cooled from a temperatureimmediately above the A transformation temperature at a rate describedby curve 1 (FIG. 3) are each subjected to a submerged arc weldingoperation wherein they are rapidly heated to a temperature of 1350 C.and subsequently cooled each at a different rate. The samples aredesignated A, B, C and D, respectively, and the time at which coolingbegins is considered the zero time (FIG. 4). The cooling curves for thefour samples are superimposed as a band upon the phase diagram of thesteel (FIG. 5). The line a-a' corresponds to the cooling curve forsample A and the line dd corresponds to the cooling curve for sample D.The area between line 11-11 and line d-d' is illustrated as a hatchedband to represent any number of cooling curves, such as that for sampleB and that for sample C, therebetween. It is noted that in such an areaa cooling time from the A transformation point to 500 C. range from 5 to50 seconds while a cooling time from 500 to 200 C. range from to 4,000seconds. Thus, it is seen that, according to the invention, cooling froma temperature about the A transformation temperature after the weldingoperation is conducted at a rate approximating the rate of cooling froma temperature about the A transformation temperature before the weldingoperation. This is readily appreciated by noting the similarity in shapebetween cooling curve 1 (FIG. 3) and the area defined by the coolingcurves for samples A and D (FIG. 5). The term rate is employed herein todesignate the continuum of rates which can best be illustrated by acooling curve.

It is essential to the invention that cooling between the Atransformation temperature and the temperature at which transformationto martensite begins (M be ap' proximately the same in the preliminarytreating step (FIG. 3) as in the cooling subsequent to welding (FIG. 5).Furthermore, it is preferred for the best results that in both thepreliminary treatment and subsequent to welding the steel be cooled fromabout the temperature at which transition from austenite to martensitebegins toward room temperature at rates approximately the same, and lessthan the rates of cooling used in cooling from the temperature A totemperature M If desired, after a preliminary treatment according to theinvention and before the welding operation, the duc tility of the steelmay be increased by the formation of spheroidic carbide by means oftempering the steel at temperatures from about 150 C. to immediatelybelow the A transformation temperature, the higher temperatures in thisrange affording greater degrees of ductility.

The type B steel preliminarily treated and cooled subsequent to weldingaccording to the present invention demonstrates little of theconventional embrittlement caused by welding. This is illustrated by thefact that the Charpy impact value of the steel after the Weldingoperation is still high.

Particularly good results are obtained according to the presentinvention when the alloy steel is fairly high in its content of alloyelements, i.e., at least 3 or 4% by weight. In conventional steelterminology, carbon is not considered an alloy element because itspresence is required to have any steel at all. An example of a ratherhigh alloy steel is the following one, of which the composition isindicated in terms of percentage by weight in Table 2 and the propertiesof which are indicated in Table 3.

The steel of Tables 2 and 3 is subjected to a preliminary treatmentaccording to the invention like that illustrated in FIG. 3 with respectto another steel. The Charpy impact value of this preliminarily treatedsteel according to the present invention and of the conventionallytreated type B steel are measured at temperatures between about C. and+20 C. It is found that a curve In which represents the Charpy impactvalues for the preliminarily treated steel according to the presentinvention is at approximately the same level as a curve h whichrepresents the Charpy impact values for the type B steel (FIG. 6),whereby it is demonstrated that preliminarily treated steel according tothe present invention has a toughness comparable to the toughness ofconventionally treated steel of the prior art.

Then the steel of Tables 2 and 3 is subjected to a welding operation inwhich a portion of the steel is heated to a temperature as high as 1350C. After the welding operation this steel is cooled in accordance withthe invention at the same rate starting at the A transformationtemperature as illustrated in FIG. 5 for another steel. Then the Charpyimpact values of areas of this steel which have been heated by thewelding operation to various temperatures up to 1350 C. is compared withthe Charpy impact values of like areas of the conventionally treatedtype B steel which has been subjected to a like welding operation. TheCharpy impact values for the conventionally treated type B steel whichhave been illustrated in FIG. 1 are illustrated again in FIG. 7 whereinthe curve representing them is designated curve 1'. A curve i, whichrepresents the Charpy impact values of the steel of Tables 2 and 3 whichhas been preliminarily treated and subjected to welding followed bycooling according to the present invention, does not exhibit the verypronounced downturns exhibited by the curve 1'. Thus, the steel treatedand welded according to the present invention essentially lacks theembrittlement found in the conventionally heat treated and welded steel.Thus, according to the present invention, embrittlement of alloy steelupon welding, which has been particularly severe in areas of the steelheated to a maximum temperature within approximately 100 C. of the Atransformation temperature or heated above the A transformationtemperature (FIG. 1) is avoided.

The interval during which cooling from about 800 C. (i.e., about the A,-transformation temperature) to about 500 C. (i.e., about the beginningof the transition from austenite to martensite) is effected after thewelding operation is found to be very significant with respect to theCharpy impact value of conventionally treated alloy steels. It isdisclosed above that the Charpy impact value of the conventionallytreated type B steel decreases very sharply as the time interval isincreased to about 40 seconds and, then, finally levels oflf (FIG. 2).The cooling curve of FIG. 2 is reproduced as curve I in FIG. 8. A curvek, which represents the effect of the time interval upon the Charpyimpact value of the steel of Tables 2 and 3 preliminarily treated andcooled after welding according to the present invention, indicates agradual, rather than a sharp, decrease. This is another advantage of thepresent invention in that it permits more latitude in selecting rates ofcooling.

In the preliminary treatment of the present invention, preferably thetemperature of the steel is held immediately above the A transformationtemperature until uniform austenite structure has formed throughout thesteel. Generally, a convenient point for applying this treatment isafter the blooming and rolling of the ingot. It is found that thepreliminary treatment of the invention results in the ultimate formationmainly of bainite structure. Appatently, by virtue of the formation ofthe bainite structure it is possible to obtain alloy steels which resistembrittlement upon welding. Furthermore, frequently in the prior art anextremely high alloy steel was employed in order that the steel wouldstill be reasonably tough even after being embrittled by the weldingoperation. However, such extremely high alloy steels tended to be undulyhard and, therefore, susceptible to stress corrosion cracking in thepresence of sulfides. Thus, this problem would arise, for example, inWelded steel vessels for propane gas since propane gas containssulfides. This problem is entirely avoided by the present invention.

It Will be appreciated that this invention is particularly useful in thecase of automatic welding such as submerged arc welding and carbondioxide arc welding, because in such welding the heat input to the steelis particularly great and, therefore, the embrittlement problem isparticularly severe.

The method of the present invention is particularly useful in anycontext in which it is desired to automatically weld high strength,i.e., alloy, steel, such as shipbuilding, and the construction ofbridges, pressure vessels, tanks, rocket chambers, and the like.

The invention is not to be construed as limited to the particularembodiments disclosed herein, since these are to be regarded asillustrative rather than restrictive.

What I claim and desire to secure by Letters Patent is:

1. A method of pre-treating and welding alloy steel comprising heatingthe steel to a temperature immediately above the A transformationtemperature of the steel, then cooling the steel at a first cooling rateto approximately a temperature at which martensitic transformation ofthe steel begins, and finally cooling the steel to room temperature,then subjecting the steel to a welding operation, wherein the portion ofsteel heated by the welding operation is heated to a temperature aboveabout the A transformation temperature, cooling said portion of steelafter the welding operation at a rate substantially the same as said,first cooling rate between approximately the A transformationtemperature and approximately a temperature at which martensitictransformation begins, and then cooling said portion of steel to roomtemperature at a slower rate whereby embrittlement of the steel by thewelding operation is essentially avoided.

2. A method according to claim 1, in which after both of said coolingsteps carried out at said first cooling rate, the steel is cooled fromapproximately a temperature at which transition from austenite tomartensite begins toward room temperature at rates approximately thesame, and less than the rates of cooling used to cool the steel fromsaid A transformation temperature to the beginning of said martensitictransformation.

3. A method of heat treating alloy steel which comprises the steps ofheating the steel above the A transformation temperature of the steel,cooling the steel from said A transformation temperature to about 500 C.for a period of time ranging from about 5 to 50 seconds, and thencooling the steel from about 500 to 200 C. for a period of time rangingfrom about 20 to 4,000 seconds and finally cooling the steel to roomtemperature, then subjecting said steel to welding whereby the portionof steel heated by the welding operation is heated to a temperatureabove about the A transformation temperature and then cooling saidportion of steel to room temperature, said first and second coolingsteps prior to said welding substantially following the cooling curvefollowed by the resultant weld, whereby embrittlement of the steel issubstantially eliminated.

4. A method as claimed in claim 3 wherein after the steel has beensubjected to the cooling treatment as defined in claim 3, the steel istempered. at temperatures from about C. to below the A transformationtemperature.

5. A method as claimed in claim 3 wherein the steel is transformed fromthe austenitic into the bainitic structure.

References Cited UNITED STATES PATENTS 2,224,998 12/1940 Wood et al.148-153 2,441,628 5/1948 Grifiiths et al. 148143 2,732,323 1/1956Linnert 148127 3,103,065 9/1963 Rectenwald 148-127 X 3,111,436 11/1963McGavin 148-143 3,192,079 6/1965 Takagi et al. 14834 X CHARLES N.LOVELL, Primary Examiner US. Cl. X.R. 29498

